MdePkg/UefiDevicePathLib: Fix the wrong MAC address length

MdePkg/UefiDevicePathLib: Fix the wrong MAC address length

Network interface type should be checked before the conversion between
text device path node and MAC device path. Otherwise, the MAC text string
can't be converted to the representation of a device node, which leads to
the series failure of network HII configuration(e.g. IP, VLAN, HTTP Boot
configuration in Network Device List).

Cc: Liming Gao <liming.gao@intel.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Wu Jiaxin <jiaxin.wu@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Ye Ting <ting.ye@intel.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>